Mr. Shanil N. Virani Profile

Shanil N. Virani

Graduate Student at Yale Univ

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Author

Publications (12)

Proceedings Article | 18 August 2005 Paper

Managing radiation degradation of CCDs on the Chandra X-ray Observatory II

Stephen O'Dell, Thomas Aldcroft, Bradley Bissell, William Blackwell, Robert Cameron, Jon Chappell, Joseph DePasquale, Kenneth Gage, Catherine Grant, Christine Harbison, Michael Juda, Kevin Marsh, Eric Martin, Joseph Minow, Stephen Murray, Paul Plucinsky, Daniel Schwartz, Daniel Shropshire, Bradley Spitzbart, Shanil Virani, Brent Williams, Scott Wolk

Proceedings Volume 5898, 58980R (2005) https://doi.org/10.1117/12.619442

KEYWORDS: Charge-coupled devices, Sensors, X-rays, Solar processes, Observatories, Space operations, Magnetosphere, Bismuth, Solar radiation models, Solar radiation

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Proceedings Article | 11 October 2004 Paper

An evaluation of a bake-out of the ACIS instrument on the Chandra X-Ray Observatory

Paul Plucinsky, Stephen O'Dell, Neil Tice, Douglas Swartz, Marshall Bautz, Joseph DePasquale, Richard Edgar, Gordon Garmire, Rino Giordano, Catherine Grant, Perry Knollenberg, Steve Kissel, Beverly LaMarr, Richard Logan, Martin Mach, Herman Marshall, Leon McKendrick, Gregory Prigozhin, Dan Schwartz, Norbert Schulz, Dan Shropshire, Tan Trinh, Alexey Vikhlinin, Shanil Virani

Proceedings Volume 5488, (2004) https://doi.org/10.1117/12.552029

KEYWORDS: Charge-coupled devices, Contamination, Collimators, X-rays, Space operations, Observatories, Absorption, Sensors, Thermal modeling, Aerospace engineering

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Proceedings Article | 3 February 2004 Paper

A study of the seasonal variations of the Chandra X-ray Observatory radiation model

Joseph DePasquale, Shanil Virani, Daniel Schwartz, Robert Cameron, Paul Plucinsky, Stephen O'Dell, Joseph Minow, William Blackwell

Proceedings Volume 5165, (2004) https://doi.org/10.1117/12.516469

KEYWORDS: Observatories, Databases, X-rays, Space telescopes, Magnetosphere, Solar processes, Spectrometers, Charge-coupled devices, Telescopes, Data modeling

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The Chandra X-ray Observatory (CXO), launched in July of 1999, contains two focal-plane imaging detectors and two transmission-grating spectrometers. Maintaining an optimal performance level for the observatory is the job of the Chandra X-ray Center (CXC), located in Cambridge, MA. One very important aspect of the observatory's performance is the science observing efficiency. The single largest factor which reduces the observing efficiency of the observatory is the interruption of observations due to passage through the Earth's radiation belts approximately every 2 2/3 days. During radiation belt passages, observations are suspended on average for over 15 hours and the Advanced CCD Imaging Spectrometer (ACIS) is moved out of the focus of the telescope to minimize damage from low-energy (100-200 keV) protons. The CXC has been using the National Space Science Data Center's "near Earth" AE-8/AP-8 radiation belt model to predict the entry and exit from the radiation belts. However, it was discovered early in the mission that the AE-8/AP-8 model predictions were inadequate for science scheduling purposes and a 10ks "pad time" was introduced on ingress and egress of perigee to ensure protection from radiation damage. This pad time, totaling 20 ks per orbit, has recently been the subject of much analysis to determine if it can be reduced to maximize science observing efficiency. A recent analysis evaluating a possible correlation between the Chandra Radiation Model (CRM) and the Electron Proton Helium Instrument (EPHIN) found a greatest lower bound (GLB) in lieu of a correlation for the ingress and egress of each perigee. The GLB is a limit imposed on the CRM such that when the CRM exceeds this limit on ingress, this defines the new safing time and similarly for egress. We have shown that using this method we can regain a significant amount of lost science time at the expense of minimal radiation exposure. The GLB analysis also found that different GLB's produce varied results and hint that there could be a time dependence associated with the GLB, possibly related to the orientation of the Observatory's orbit. Utilizing CRM V2.3, we present the search for a seasonal dependence on the value of the GLB; we find a seasonal effect that appears to depend on the orientation of Chandra's orbit with respect to the Earth's magnetic field.

Proceedings Article | 3 February 2004 Paper

Improving the science observing efficiency of the Chandra X-ray Observatory via the Chandra radiation model

Shanil Virani, Joseph DePasquale, Daniel Schwartz, Robert Cameron, Paul Plucinsky, Stephen O'Dell, Joseph Minow, William Blackwell

Proceedings Volume 5165, (2004) https://doi.org/10.1117/12.505878

KEYWORDS: Data modeling, Observatories, X-rays, Instrument modeling, Magnetosphere, Charge-coupled devices, Satellites, Databases, Magnetism, Solar processes

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The Chandra X-ray Observatory (CXO), NASA's latest "Great Observatory", was launched on July 23, 1999 and reached its final orbit on August 7, 1999. The CXO is in a highly elliptical orbit, with an apogee altitude of 120,000 km and a perigee altitude 20,000 km, and has a period of approximately 63.5 hours (≈ 2.65 days). It transits the Earth's Van Allen belts once per orbit during which no science observations can be performed due to the high radiation environment. The Chandra X-ray Observatory Center currently uses the National Space Science Data Center's "near Earth" AP-8/AE-8 radiation belt model to predict the start and end times of passage through the radiation belts. Our earlier analysis (Virani et al, 2000) demonstrated that our implementation of the AP-8/AE-8 model (a simple dipole model of the Earth's magnetic field) does not always give sufficiently accurate predictions of the start and end times of transit of the Van Allen belts. This led to a change in our operating procedure whereby we "padded" the start and end times of transit as determined by the AE-8 model by 10 ks so that ACIS, the Advanced CCD Imaging Spectrometer and the primary science instrument on-board Chandra, would not be exposed to the "fringes" of the Van Allen belts on ingress and egress for any given transit. This additional 20 ks per orbit during which Chandra is unable to perform science observations integrates to approximately 3 Ms of "lost" science time per year and therefore reduces the science observing efficiency of the Observatory. To address the need for a higher fidelity radiation model appropriate for the Chandra orbit, the Chandra Radiation Model (CRM) was developed. The CRM is an ion model for the outer magnetosphere and is based on data from the EPIC/ICS instrument on-board the Geotail satellite as well as data from the CEPPAD/IPS instrument on-board the Polar satellite. With the production and implementation of the CRM Version 2.3, we present the results of a study designed to investigate the science observing time that may be recovered by using the CRM for science mission planning purposes. In this paper, we present a scheme using the CRM such that for a modest increase in ACIS CTI, approximately 500 ks can be recovered each year for new science observations.

Proceedings Article | 11 March 2003 Paper

Flight spectral response of the ACIS instrument

Paul Plucinsky, Norbert Schulz, Herman Marshall, Catherine Grant, George Chartas, Divas Sanwal, Marcus Teter, Alexey Vikhlinin, Richard Edgar, Michael Wise, Glenn Allen, Shanil Virani, Joseph DePasquale, Michael Raley

Proceedings Volume 4851, (2003) https://doi.org/10.1117/12.461473

KEYWORDS: Charge-coupled devices, Calibration, Data modeling, Contamination, Spectral resolution, Absorption, Bismuth, Manganese, X-rays, Sensors

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We discuss the flight calibration of the spectral response of the Advanced CCD Imaging Spectrometer (ACIS) on-board the Chandra X-ray Observatory (CXO). The spectral resolution and sensitivity of the ACIS instrument have both been evolving over the course of the mission. The spectral resolution of the frontside-illuminated (FI) CCDs changed dramatically in the first month of the mission due to radiation damage. Since that time, the spectral resolution of the FI CCDs and the Backside-illuminated (BI) CCDs have evolved gradually with time. We demonstrate the efficacy of charge-transfer inefficiency (CTI) correction algorithms which recover some of the lost performance. The detection efficiency of the ACIS instrument has been declining throughout the mission, presumably due to a layer of contamination building up on the filter and/or CCDs. We present a characterization of the energy dependence of the excess absorption and demonstrate software which models the time dependence of the absorption from energies of 0.4 keV and up. The spectral redistribution function and the detection efficiency are well-characterized at energies from 1.5 to 8.0~keV primarily due to the existence of strong lines in the ACIS calibration source in that energy range. The calibration at energies below 1.5 keV is challenging because of the lack of strong lines in the calibration source and also because of the inherent non-linear dependence with energy of the CTI and the absorption by the contamination layer. We have been using data from celestial sources with relatively simple spectra to determine the quality of the calibration below 1.5 keV. We have used observations of 1E0102.2-7219 (the brightest supernova remnant in the SMC), PKS2155-304 (a bright blazar), and the pulsar PSR~0656+14 (nearby pulsar with a soft spectrum), since the spectra of these objects have been well-characterized by the gratings on the CXO. The analysis of these observations demonstrate that the CTI correction recovers a significant fraction of the spectral resolution of the FI CCDs and the models of the time-dependent absorption result in consistent measurements of the flux at low energies for data from a BI (S3) CCD.

Proceedings Article | 11 March 2003 Paper

Managing radiation degradation of CCDs on the Chandra X-ray Observatory

Stephen O'Dell, William Blackwell, Robert Cameron, Joseph Minow, David Morris, Bradley Spitbart, Douglas Swartz, Shanil Virani, Scott Wolk

Proceedings Volume 4851, (2003) https://doi.org/10.1117/12.461493

KEYWORDS: Charge-coupled devices, X-rays, Solar radiation models, Observatories, Magnetosphere, Back illuminated sensors, Solar processes, Spectroscopy, Space telescopes, Solar radiation

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Proceedings Article | 2 January 2002 Paper

Monitoring the health and safety of the ACIS instrument on board the Chandra X-ray Observatory

Shanil Virani, Peter Ford, Joseph DePasquale, Paul Plucinsky

Proceedings Volume 4844, (2002) https://doi.org/10.1117/12.460672

KEYWORDS: Safety, Observatories, Space operations, X-rays, Charge-coupled devices, Data communications, Sensors, Spectroscopy, Actuators, Analog electronics

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Proceedings Article | 2 January 2002 Paper

Optimizing the efficiency of command load inspection for the Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Telescope

Joseph DePasquale, Shanil Virani, Paul Plucinsky

Proceedings Volume 4844, (2002) https://doi.org/10.1117/12.460673

KEYWORDS: Space operations, Observatories, Inspection, Lawrencium, Spectroscopy, X-rays, Safety, Charge-coupled devices, X-ray telescopes, Sensors

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Proceedings Article | 13 December 2000 Paper

Modeling the Chandra space environment

William Blackwell, Joseph Minow, Stephen O'Dell, Robert Suggs, Douglas Swartz, Allyn Tennant, Shanil Virani, Kevin Warren

Proceedings Volume 4140, (2000) https://doi.org/10.1117/12.409105

KEYWORDS: Ions, Solar radiation models, Magnetosphere, Solar processes, Databases, Satellites, Space operations, Sensors, Data modeling, Plasma

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Proceedings Article | 13 December 2000 Paper

Radiation environment of the Chandra X-Ray Observatory

Stephen O'Dell, Mark Bautz, William Blackwell, Yousaf Butt, Robert Cameron, Ronald Elsner, M. Gussenhoven, Jeffery Kolodziejczak, Joseph Minow, Robert Suggs, Douglas Swartz, Allyn Tennant, Shanil Virani, Kevin Warren

Proceedings Volume 4140, (2000) https://doi.org/10.1117/12.409104

KEYWORDS: Charge-coupled devices, X-rays, Observatories, Solar radiation models, Space operations, Ions, Back illuminated sensors, Space telescopes, Mirrors, Nanoimprint lithography

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Proceedings Article | 18 July 2000 Paper

Chandra X-ray Observatory's radiation environment and the AP-8/AE-8 model

Shanil Virani, Reinhold Mueller-Mellin, Paul Plucinsky, Yousaf Butt

Proceedings Volume 4012, (2000) https://doi.org/10.1117/12.391606

KEYWORDS: Sensors, Data modeling, Observatories, X-rays, Particles, Solar energy, Magnetism, Magnetosphere, Charge-coupled devices, Wind energy

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Proceedings Article | 18 July 2000 Paper

Observed on-orbit background of the ACIS instrument on the Chandra X-ray Observatory

Paul Plucinsky, Shanil Virani

Proceedings Volume 4012, (2000) https://doi.org/10.1117/12.391607

KEYWORDS: Charge-coupled devices, Particles, Bismuth, Fourier transforms, X-rays, Observatories, Sensors, Information operations, Image enhancement, Spectroscopy

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Showing 5 of 12 publications

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